I pick my broodies up and check under them every day until the eggs hatch. A dedicated brooder with strong broody instincts won't stop just because you pick her up. It also gives you chance to check for broken eggs, and clean up the other eggs and replace bedding in the nest if any did break.
I've seen hens successfully hatch eggs when I was sure they were all dead from being repeatedly chilled, (I had a hen that could never remember the right nest) from being washed off two or three times during incubation because of broken eggs in the nest, etc. Then, at 21 days, there were chicks, sometimes to my great surprise! It's amazing what some of them will survive. Then other times, you lose some for no apparent reason.
From 18 inches, if there's just a nice pile of loose hay underneath for them to land on, they should be fine. But they won't be able to get back up there, mom will need an alternate spot for nighttime.
ETA: I went back and saw you have a 'bator, if you have a 3 day spread on the eggs she still has, if there are some still in the nest when she takes the first chicks out, you might stick them in the bator. I'd set it up a couple of days before the eggs are due to start hatching, to get the temp adjusted right. The worst that'll happen is the won't hatch. If they do, you might be able to sneak them in with mom at night. That's kinda of a crapshoot, some will accept more chicks, some won't. I have one that'll adopt anything, even when her clutch is half grown, but that's unusual.